In recent years, a research on an optical communication based on a digital coherent system has been advanced to cope with an increase in communication traffics. A waveform distortion correction, an adaptive equalization, and the like are carried out by a digital signal processing circuit in a digital coherent receiver, and high characteristics can be obtained even in a transmission at a high bit rate. In a case where a dual polarization-quadrature phase shift keying (DP-QPSK) modulation system is employed, two-bit data may be allocated to four modulated optical phases (0°, 90°, 180°, and 270°) with regard to each of two orthogonal polarizations, and a symbol speed may be reduced to a quarter of the original speed.
The received optical signal is subjected to a photoelectric conversion and an analog/digital conversion, and waveform distortion components and the like are adaptively equalized by an adaptive equalizer in the digital signal processing circuit.
The adaptive equalizer illustrated in FIG. 1 is composed of a butterfly finite impulse response (FIR) filter and adapted to perform a separation of a polarized orthogonal multiple signal, a polarization mode dispersion compensation, and the like. A constant modulus algorithm (CMA) system is used for an adaptive control on tap coefficients of the individual filters, for example. A convergence state is unchanged even when all the tap coefficients on an H side (or all the tap coefficients on a V side) in units of one symbol (in units of the tap number corresponding to one symbol) are shifted by the FIR filter. This is because, when all the tap coefficients are shifted at once, only the absolute time is changed, and the relationship is maintained. In a case where twofold oversampling data is processed, a gravity center location of the tap coefficients in units of two taps (one symbol) is adjusted by shifting the entire tap coefficients so that the signal communication can be resumed without a second pull-in.
In a case where the weight of the tap coefficients is deviated to an end part of the taps, for example, a case where the coefficient value of the tap number 1 or the tap number 13 is high in the 13-tap FIR filter, the equalization residual of the adaptive equalizer is generated, and the signal degradation is caused. For that reason, the weight of the tap coefficients is desirably shifted towards a center of the taps as much as possible.
A specific correction method of shifting the gravity center of the tap coefficients towards the tap center as much as possible is proposed (for example, see Japanese Laid-open Patent Publication No. 2012-119923). The gravity center value of the tap coefficients is calculated, and the tap coefficients are shifted in units of one symbol so that the gravity center of the coefficients is at the center of the taps (in a range between the tap number 6 and the tap number 8 in the case of the 13-tap filter). That is, in a case where a horizontal axis represents the tap number and a vertical axis represents the tap coefficient, a tap number or a nearest tap number with which the area of a drawn figure is halved is referred to as “gravity center of the tap coefficients”. The gravity center of the tap coefficients is calculated with respect to each of the H polarization and the V polarization, and the correction of shifting the gravity center of the tap coefficients towards the center of the taps is also conducted with respect to each of the H side and the V side.
According to the related art method, to carry out the correction of shifting the gravity center of the tap coefficients towards the tap center, an instantaneous value representing the tap coefficient gravity center at that moment is used. According to this method, an optimal correction is not carried out depending on a state of the polarizations, and a correction effect is not attained to a maximum extent. As a result, a differential group delay (DGD) resistance is decreased, and a bit error is likely to increase.